1. Field of the Invention
The present invention relates to a linear motor used for transporting a card such as an optical card.
2. Description of the Related Art
A linear motor has been used for transporting a card such as an optical card or driving an optical head or the like.
A linear motor is constructed of a stator and a rotor or movable member. The stator has a pair of magnetic circuits each constructed of a linear yoke and another yoke. Within the spaces defined by the two linear yokes and the other two yokes, a pair of linear magnets are mounted on the other two yokes while providing linear gaps between the magnets and the linear yokes. The rotor has a moving coil and is mounted on the linear yokes constituting the magnetic circuits. When the moving coil is powered, the rotor linearly moves along the linear yokes.
Referring to FIGS. 34 and 35 showing an example of a conventional linear motor, linear magnets 1 and 2 magnetized in the thickness direction are mounted on the inner surfaces of yokes 3 and 4 with the same poles facing each other, the yokes having both end portions being bent. Linear yokes 5 and 6 inserted into a bobbin 8 are integrally coupled to the yokes 3 and 4. The bobbin 8 has a moving coil 7 wound about it.
The moving coil 7 is mounted to a holding member 9 formed in the lower portion of a shuttle 11 which extends over the moving coil, yokes, and magnets. The shuttle 11 has bearings 12 and 13 at opposite sides thereof. Guide shafts 14 and 15 are coupled to the bearings 12 and 13.
With the linear motor constructed as above, magnetic fluxes from the N poles of the linear magnets 1 and 2 go toward the inner linear yokes 5 and 6 while traversing the moving coil 7, and return back via the linear yokes 5 and 6 to the S poles of the linear magnets 1 and 2. If a d.c. current flows through the moving coil 7, the magnetic fluxes of the linear magnets 1 and 2 link with the d.c. current to provide an electromagnetic action therebetween so that a mechanical action force is generated between the linear magnets 1 and 2 and the moving coil 7 to thereby move the latter.
A thrust force F applied to the moving coil 7 is generally given by F =Bil, where B is a magnetic flux density acting to the moving coil, i is a coil current, and 1 is an effective coil length activated by a magnetic field. In order to move the shuttle 11 (moving coil 7) in the reverse direction, the current supplied to the moving coil 7 is reversed.
In a transport apparatus for transporting a card such as an optical card by using a linear motor of the type described above, it is necessary to reciprocally move an optical card at a high speed, and to decelerate, stop, and accelerate the card within a limited distance or in a limited time.
A conventional transport apparatus is associated, however, with some problem. Namely, magnetic fluxes from the permanent magnets for generating magnetic fields in the linear gaps in the same direction are uniform, i.e., the permanent magnets each are formed with a single magnet constituting a magnetic circuit. As a result, if an acceleration speed (deceleration speed) is made faster, magnetic saturation occurs at the opposite end portions of the linear yokes 5 and 6 and other yokes 3 and 4, so that the magnetic flux density reduces at the acceleration speed (deceleration speed) area and a desired acceleration speed (deceleration speed) cannot be obtained.
Another conventional linear motor is shown in FIG. 36 wherein a single magnet is used to form a magnetic circuit. The magnetic flux density distribution gradually changes in the longitudinal direction of the motor. It is therefore necessary to flow a very large current in order to obtain a sufficient thrust force at the acceleration speed area, resulting in a large diameter of coil winding and a large weight of the driving system.
Another conventional linear motor is shown in FIG. 37 wherein yokes 3, 4, 5, and 6 constituting a magnetic circuit are designed to have a sufficiently small magnetic reluctance. Although this linear motor has a good magnetic characteristic, the thickness of a yoke becomes large, resulting in a large-sized magnetic circuit.
In a conventional optical head driving mechanism using linear motors shown in FIGS. 38 and 39, the centers of guide shafts are not flush with the plane of driving force of the linear motors serving as driving means. This is also true for the case of a card transport mechanism. In FIGS. 38 and 39, two parallel guide shafts 2' and 3' are disposed different in height at both sides of a transport stage 1' having an optical head. The guide shafts 2' and 3' are inserted into two slide bearings 4' such that the transport stage 1' can be guided along the guide shafts 2' and 3'.
Moving coil type linear motors are provided at both sides of the transport stage 1'. An elongated rectangular yoke 5' is disposed above and along the guide shaft 2'. A permanent magnet 6' magnetized in the thickness direction thereof is mounted at the yoke 5' at the position remotely from the transport stage 1'. The yoke 5' is inserted into the central hole of a coil 7' which is fixedly connected to the transport stage 1'. One of the linear motors is constituted by the yoke 5', permanent magnet 6', and coil 7'.
An encoder plate 12' mounted on the upper surface of the transport stage 1' and a sensor 13' disposed at the back of the yoke 5' constitute a linear encoder for detecting the position and speed of the transport stage 1'.
As described above, with this conventional optical head driving mechanism, the centers of the guide shafts 2' and 3' are not flush with the plane of driving force of the linear motors, a difference in height being present between them.